Enter the collection area, rainfall intensity, and collection efficiency into the calculator to determine the amount of rainwater collected; this calculator can also evaluate any of the variables given the others are known.

Rain Collection Formula

The following formula is used to calculate the amount of rainwater collected by a rain collection system:

RC = A * R * C


  • RC is the amount of rainwater collected
  • A is the collection area (in square meters)
  • R is the rainfall intensity (in millimeters per hour)
  • C is the collection efficiency (a decimal value between 0 and 1)

To calculate the amount of rainwater collected, multiply the collection area by the rainfall intensity, and then multiply the result by the collection efficiency.

What is a Rain Collection?

Rain collection, also known as rainwater harvesting, is a process that involves gathering, storing, and managing rainwater for future use. This is typically done by collecting rainwater from a roof or other hard surface, such as a patio or driveway, and directing it into a storage container, such as a barrel or cistern, through a system of gutters and downspouts. The collected rainwater can then be used for various purposes, such as watering plants, flushing toilets, washing clothes, or even drinking, provided it has been properly treated and purified. Rain collection is an effective way to conserve water, reduce water bills, and lessen the demand on municipal water supplies. It also helps to reduce stormwater runoff, which can lead to erosion and flooding.

How to Calculate Rain Collection?

The following steps outline how to calculate the Rain Collection:

  1. First, determine the area of the collection surface (A) in square meters.
  2. Next, determine the average rainfall (R) in millimeters.
  3. Next, convert the average rainfall from millimeters to meters by dividing by 1000.
  4. Next, calculate the volume of water collected (V) in cubic meters using the formula V = A * R.
  5. Finally, calculate the Rain Collection by multiplying the volume of water collected by the density of water (D) in kilograms per cubic meter.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

area of the collection surface (A) = 50 square meters

average rainfall (R) = 100 millimeters

density of water (D) = 1000 kilograms per cubic meter